Petroleum tankers were heretofore filled with petroleum into a hole in the upper portion of the tank, wherein the fullness is determined visually. Due to regulations, the tankers have now been required to be filled through a hole in the bottom of the tank, the same hole from which the fuel is emptied. Therefore, sensors are required on the upper portion of the tank to indicate when the tank is full since visual determination is no longer possible. Generally, the sensor will be directly connected to the filling facility or station, whereby a signal is produced by the sensor when a liquid is detected to cease filling from the facility. A plurality of sensors have been used, mainly thermistors, float switches, or fiber optic probes. The float switches and thermistors have been ineffective because of mechanical and electrical problems and because truck drivers are able to tamper with and disconnect them. A problem with the fiber optic probe is that it needs to be cleaned occasionally, otherwise it would always indicate the presence of the liquid.
One type of prior art capacitor sensor assembly includes a capacitive probe and a reference capacitor connected at a common point. Two different potential signals are supplied to each the capacitive probe and reference capacitor. Any excursions of potential at the common point indicates the status of the capacitive probe, whether it has detected a liquid or not. Such an assembly is shown is U.S. Pat. No. 4,107,658 in the name of William Frank Hill granted on Aug. 15, 1978 which discloses a capacitive sensing system which includes a reference capacitor and capacitive probe with two different potential signals applied to produce an excursion at a common point indicating the state of the capacitive probe. The problem with this type of assembly is that it is not completely temperature compensative.